1. Field of the Invention
The present invention relates to a catalyst for polymerizing olefins, particularly alpha-olefins, a method for producing such a catalyst and to a polymerization process utilizing the catalyst which produces polymer products having narrow molecular weight distribution. A particular aspect of the present invention relates to a method for preparing a supported catalyst composition which produces polymers whose melt flow ratio (MFR) decreases with the decreasing catalyst average pore size at a substantially constant catalyst particle size. MFR, as is known to those skilled in the art, is an indication of polymer's molecular weight distribution. Lower MFR values indicate narrower molecular weight distribution of the polymer than higher MFR values.
2. Description of Related Art
Linear low density polyethylene polymers possess properties which distinguish them from other polyethylene polymers, such as ethylene homopolymers. Certain of these properties are described by Anderson et al, U.S. Pat. No. 4,076,698.
Karol et al, U.S. Pat. No. 4,302,566, describe a process for producing linear low density polyethylene polymers in a gas phase, fluid bed reactor.
Graff, U.S. Pat. No. 4,173,547, Stevens et al, U.S. Pat. No. 3,787,384, Strobel et al, U.S. Pat. No. 4,148,754, and Ziegler, deceased, et al, U.S. Pat. No. 4,063,009, each describe various polymerization processes suitable for producing forms of polyethylene other than linear low density polyethylene, per se.
Stevens et al, U.S. Pat. No. 3,787,384, Strobel et al, U.S. Pat. No. 4,148,754, and Ziegler, deceased, et al, U.S. Pat. No. 4,063,009, describe various olefin polymerization catalysts.
Nowlin et al, U.S. Pat. No. 4,481,301, the entire contents of which are incorporated herein by reference, disclose a supported alpha-olefin polymerization catalyst composition prepared by reacting a support containing OH groups with a stoichiometric excess of an organomagnesium composition, with respect to the OH groups content, and then reacting the product with a transition metal compound.
Furtek, U.S. Pat. No. 4,670,413, discloses a supported alpha-olefin polymerization catalyst composition having an average pore diameter of at least about 170 Angstroms. The productivity of the catalyst and resin melt and high load melt indices (I.sub.2 and I.sub.21, respectively) are higher than those of similar catalysts of lower average pore diameter. Furthermore, the productivity, I.sub.2 and I.sub.21, increase with increasing average pore diameter at an unspecified catalyst particle size. Similarly, polymer density decreases with increasing average pore diameter at an unspecified particle size.
McDaniel, Fracturing Silica-Based Catalysts During Ethylene Polymerization, JOURNAL OF POLYMER SCIENCE: POLYMER CHEMISTRY EDITION, Vol. 19, 1967-76 (1981), teaches that the activity of a supported titanium (Ti) and magnesium (Mg)-containing olefin polymerization catalyst increases until the average silica support pore diameter reaches 400 Angstroms (.ANG.). The particle size of the support is not specified.
Karol et al, European Published patent application No. 84 103441.6, Publication No. 0 120 503, disclose that olefin polymerization catalyst activity can be improved by using a silica support having average pore sizes of at least 80, preferably at least 100 .ANG., and an unspecified particle size.
Shida et al, POLYMER PREPRINTS (ACS Div. Poly. Chem.) 24(1), pages 110-111 (1983), teach that silica support has an effect on the molecular weight of the resins produced with the silica-supported chromium-based olefin polymerization catalysts. However, this effect is missing in the magnesium- and titanium-containing catalysts. Similarly, Hogan et al JOURNAL OF APPLIED POLYMER SCIENCE: APPLIED POLYMER SYMPOSIUM, 36, 49-60 (1981), disclose than an increase in pore size of chromium-based olefin polymerization catalysts increases melt index of the resin produced with the catalyst.
One of the important properties of blown and cast films and injection-molded articles made from olefinic polymer resins is the strength of such films and articles. As is known to those skilled in the art, resin strength properties are inversely related to the MFR values thereof. Thus, resins having lower MFR values are likely to have better strength properties than similar resins of higher MFR values. Accordingly, workers in the art continue to search for ways of producing resins having relatively low MFR values.
It is a primary object of the present invention to prepare a high activity supported catalyst for the polymerization of olefins, particularly alpha-olefins, which produces polymers having relatively low MFR values.
Additional objects of the present invention will become apparent to those skilled in the art from the following description.